Plasma induced polymerization or plasma enhanced chemical vapor deposition is a known technique of surface coating that is environmentally compatible because it allows for a solvent-free coating of objects.
Pulsed plasma polymerization is known to generate a polymer layer to repel liquids. This technique is e.g. described in EP 0 988 412 B1.
US 2009/0318609 A1 describes a continuous plasma polymerization process for applying coatings containing Nitrogen (e.g. pyridine) to a substrate to enhance adhesion and growth of biological cells.
Plasma polymerization is a process in which active species such as ions and free radicals are formed in a low pressure gas by igniting a plasma state in the gas in the presence of a monomer. It is believed that by collisions between free electrons and monomer molecules the polymerization process of the monomer is induced. The plasma is typically ignited by applying an electric field to the gas. The active species react with themselves or with monomers to form polymer coatings on the surfaces of solids that are exposed to the plasma. For plasma polymerization a plasma chamber is used wherein a low pressure gas atmosphere is created by evacuation.
Plasma polymerization takes place in a low pressure and low temperature plasma that is produced by a glow discharge in a controlled atmosphere, such as an inert gas atmosphere. An organic monomer having active elements suitable for polymerization may be present in the inert gas and/or may be deposited on the surfaces of the material to be coated. The results of coating through plasma induced polymerization depends on a large number of variables such as: monomer flow rate, system pressure and discharge power, the reactivity of the starting monomer, the frequency of the excitation signal and the temperature of the substrate and the duration of exposure. The overall power input in plasma polymerization is used for creating the plasma and for fragmentation of monomer. Plasma is a direct consequence of the ionization of the gases present in the reactor and fragmentation leading to polymerization is believed to be a secondary process.
By the pulsed plasma polymerization process disclosed in EP 0 988 412 B1 a water and oil repellant polymer layer can be obtained that exhibits a water contact angle above 90°.
The water contact angle is the angle θ at which e.g. a droplet of water of a predetermined size meets a solid surface, as illustrated in FIG. 5.
A hydrophobic surface causes a water contact angle above 90° as illustrated in FIG. 5.